The variability of selectivity coefficients, resulting from potential changes over time and the concentration ratio of primary to interfering ions, impedes many practical applications of ion-selective electrodes (ISEs). Existing theoretical interpretations of ISE selectivity are restricted by severe assumptions, such as steady state and electroneutrality, which hinder theorizing on this problem. For this reason, for the first time, the Nernst-Planck-Poisson equations are used to predict and visualize the selectivity variability over time and the concentration ratio. Special emphasis is placed on the non-Nernstian response in the measurements with liquid-ion-exchanger- and neutral-carrier-based ISEs. The conditions under which measured selectivity coefficients are true (unbiased) are demonstrated.